1. Field of the Invention
This invention relates to a package for semiconductor integrated circuits. More particularly, this invention relates to an improvement of plastic package for integrated circuits, which is provided with leads bent in the J-shape and extending over protuberances formed on each edge of a bottom side of the enclosure, and generally called PCC (Plastic Chip Carrier) or PLCC (Plastic Leaded Chip Carrier).
2. Description of the Prior Art
In order to meet the increasing demands for mounting LSI (Large Scale Integrated Circuits) at a higher density and at a lower cost on printed circuit boards for electronic equipment, there have been developed many types of packages for LSI. Among these developments, the PLCC is so promising that outlines are standardized by JEDEC (Joint Electron Device Engineering Council) entitled "PLASTIC CHIP CARRIER (PCC) FAMILY 0.050 LEADSPACING, SQUARE, SOLDER MOUNT" (hereinafter referred to as PLCC). These advantages include occupying a small space, being inexpensive, being easy to handle, having low thermal resistance, having a well-matched thermal expansion coefficient to that of the printed circuit board and having stress absorption characteristics due to the J-shaped leads. The structure of the PLCC is schematically illustrated in FIGS. 1 through 3. The main body 11 of the PLCC encapsulating a LSI with plastic resin has a shape of an essentially flat box, which has a relatively wide top and bottom and four relatively low lateral sides. In a strict sense, each lateral side generally consists of an upper-half lateral side 23' and a lower-half lateral side 23. Each half of the lateral side extends from the mid point of the above-described lateral side and is slanted back from a plane 24 of the lateral side by 3.degree. (denoted by U in FIG. 2) toward the top 11-2 and bottom 1101 of the enclosure. The lateral side plane 24 represents each lateral side of the above-mentioned flat box and is orthogonal to the top and bottom of the box. On the bottom surface 11-1 of the flat box, there are provided a plurality of protuberances 17 which are lined up along and parallel to the lateral side. The protuberances are spaced along the lateral side by 1.27 mm, the width of each protuberance along the lateral side is 0.8 mm, the width of the base of each protuberance orthogonal to the above described width is 0.7 mm, the height of the protuberances from the bottom surface 11-1 of the flat box is about 0.6 mm. Each protuberance has a first slope 20 (FIG. 2) coplanar with each slanted lower-half lateral side 23. Each protuberance has a rounded top 21 from the first slope to a second slope 22 which has an angle of approximately 25.degree. (denoted by V in FIG. 2) with respect to each lateral side plane 24. As to the structure of the leads, through the four lateral sides of the flat box, a plurality of electrically conductive leads 12 are extended from the enclosure toward the outside, while the other end of each lead is connected by a bonding wire 14 to the corresponding terminal pad of the LSI 13 encapsulated in the main body 11. Material of the leads is copper alloy having good thermal conductivity. Immediately after extending out of the lateral side, each lead is bent and extends along approximately parallel to each lateral side toward the bottom of the box. Each lead then extends along and curves like a J-shape around its associated protuberance toward the bottom 11-1 of the box. There is a second type of PLCC as an optional variation of the above described first type. The difference between the second type and the first type is that the gaps 18 between the prior art protuberances 17 are fully filled with the same plastic material to form a continuous bank 17' of a uniform height along each lateral side as shown in FIG. 6. Details of the PLCC are also disclosed in U.S. Pat. No. 4,465,898 entitled "Carrier for Integrated Circuit," John W. Orcutt, et al. issued on Aug. 14, 1984 and U.S. Pat. No. 4,495,376 entitled "Carrier for Integrated Circuit," Angus W. Hightower, et al. issued on Jan. 22, 1985.
Soldering of a PLCC onto a printed circuit board 10 is typically done by a reflow method, which has been widely used in the electronic industry. The reflow method is as follows: Solder-paste (mixture of powdered solder, soldering flux and binder) is printed on the specific portions of printed copper leads 16 by a silk screen method. Many types of the solder-paste are available on the market. Among them, solder of 63Sn-37Pb alloy is the most popularly used. As for the flux, a rosin type is typically used. To control clearing chlorine contents in the flux, MILRMA type is preferably used. The thickness of the printed solder paste is generally 200-300 microns. A PLCC is placed on a printed circuit board for each roundly formed lead of the PLCC to sit exactly on each corresponding deposit of printed solder-paste. The printed circuit board, carrying the pasted PLCC thereon, is heated by a heating means, such as an infrared lamp or vapor-phase. The printed circuit board is heated enough for the solder paste to melt, for example 200.degree.-235.degree. C. for 40 sec maximum if infrared lamp heating is used. The printed circuit board, the PLCC and the melted solder are cooled to solidify the solder. The solidified solder is denoted by 15 in FIG. 1.
Next, in order to remove undesirably remaining flux on the printed circuit board and the PLCC, they are washed by dipping or boiling in a solvent, such as tri-chloroethylene, freon, etc. If removal of the flux is not sufficient, the remaining flux may corrode the leads 16 of printed circuit board or the leads 12 of the PLCC resulting in its disconnection. The leads are made of copper or copper alloy either of which is very susceptible to the acid in the flux. Therefore, the leads must be completely washed. However, a gap 25 between the curved lead and the associated protuberance is located just behind the soldered portion 15 of the lead and the flux is likely to enter there even though the gap is narrow, because the flux is likely to splash there due to exploding caused by the heat from soldering. Therefore, the first type of PLCC is provided with gaps, i.e. channels 18 between protuberances as the path for the flowing solvent so that the solvent can smoothly flow to and from the soldered portion 15, and the adjoining narrow gap 25. Therefore, as to the first type of PLCC, there is no problem in removing the flux.
However, there are some problems in the prior art types of PLCC. One problem with the first type of PLCC is that the protuberances are so thin in width that they are easy to break, because during the ejection of the hardened plastic resin from the molding cavities there may be produced a chip or crack on the protuberance. In particular, many of the PLCCs are processed for plastic molding at the same time in order to improve production efficiency. Side walls 23 of the protuberances 17 which face each other are parallel and orthogonal to the bottom surface 11-1 of the PLCC (FIG. 4) where 31 denotes a molding cavity. Therefore, if all the molded PLCCs are ejected exactly parallel along the side walls 23 from the mold cavities 31, no lateral stress is imposed against the side walls 23 of the protuberance 17. However, in practice, some non-parallel motion of ejection of the PLCCs from the cavities is unavoidable and this non-parallel motion causes the above-described lateral stress against the side walls of the protuberances. This lateral stress may cause a crack in the protuberance. In order to ease this lateral stress in the parallel wall 23, the parallel wall 23 of each protuberance can be slanted as shown by the dotted lines in FIG. 5 where 31' denotes a molding cavity. However, if the slanted walls are designed to provide a relatively wide bottom portion of the channel 18, the slanted walls are likely to have a sharp edge at the top of the protuberance as shown by the dotted lines A, allowing the sharp top to crack easily and which can easily hurt the fingers of a handler. If the slanted walls are designed to provide a relatively wide top of the protuberance, the slanted walls are likely to make the bottom of the channel 18 narrow as shown by the dotted lines B. Therefore the flux remaining in the channel can not be easily removed. Summing up the problems described above about the first type of PLCC, determination of the shape of the protuberance is very delicate and difficult.
A problem with the second type of PLCC, in which the channel 18 between adjacent protuberances 17 is fully filled in order to strengthen the breakable weak protuberances, is that the solvent can not smoothly flow to and from the flux remaining in the gap 25 (FIG. 6) which is about 70 microns between the curved lead and the top 25 of the protuberance 17'. Thus, the remaining flux can not be completely removed. Therefore, a PLCC having a structure satisfying the requirements of both mechanical strength of the protuberances and completeness of flux removal has been seriously in demand.